(a) Field of the Invention
This invention relates to electrically conductive grounding straps or tethers which are attached to the body by way of the individual's arms, legs or clothing in order to ground operating personnel and prevent build-up of static charges on such persons. More particularly, this invention pertains to a monitoring system for insuring that electrical continuity exists between such persons and ground with means for sounding an alarm and/or providing a visual signal whenever an open circuit or break is detected between the human body and the tether to earth. This invention is especially concerned with the use of oscillator circuitry for generating A.C. output voltages adapted to actuate an alarm in combination with an impedance network (either in the tuned circuit of the oscillator or in the load circuit thereof) to which the distal end of the tether is connected and which provides a path to ground for the body through the resistance or inductance of such impedance. The monitor of the present invention incoporates body capacitance and resistance as components of the impedance network. By way of adjustable elements in the circuit's impedance network, the impedance can be varied so that the A.C. output voltage of the circuit may be reduced below a level which will actuate the alarm when continuity exists but when a break in continuity occurs, the output voltage will exceed the threshold level of the alarm or its input circuit and trigger the alarm.
(b) Prior Art
During the handling of sensitive electronic parts and equipment, it is essential that the electrical voltage between operating or handling personnel and the electronic objects be maintained at a minimum so as to avoid destructive breakdown of voltage sensitive components. To protect against electrical energy flow between the handler and the object being handled, every effort is made to maintain both the personnel and the electronic components at ground potential by means of electrically conductive grounding systems. These grounding systems drain off charges that might otherwise build up by charge transfer or by frictional or triboelectric forces or as a result of exposure within an electrical field. Such grounding devices commonly consist of electrically conductive straps or tethers, one end of which is tied to an arm, leg or body of the user or to conductive articles of clothing while the distal end is attached to earth usually by means of a clip or terminal via a grounded water pipe or electrical cable.
One problem which has arisen in the past occurred as a result of an unperceived open circuit or break between the tether wearer and his conductive path to earth, most frequently when the terminal clip of the tether became accidentally dislodged from its grounding post but on occasion because of discontinuity at the proximal contact end or from defects in the strap itself. Under such circumstances, the human body, no longer now being discharged through the grounding tether and representing a capacitance of perhaps 50 to 250 picofarads with respect to earth, could easily develop without his awareness a potential of up to 25,000 volts, for example.
A person so charged, but believing that charge drainoff was continuing via the tether, could during discharge cause destruction of the electronic component by dielectric breakdown or detonate an explosive environment by generating a spark.
It is therefore an object of this invention to provide in combination with an electrically conductive ground tether a monitor for detecting any open circuit or break in electrical continuity between the tether wearer and ground.
Another object of this invention is to provide a sensing apparatus for continuously monitoring ground tether continuity.
Yet another object of this invention is to provide a monitoring circuit for coupling a human body to earth through his grounding tether wherein an alarm signal will be actuated in the event of a break in electrical continuity between the tether wearer and his path to ground.
Yet still another object of this invention is to provide a ground tether fault detector employing oscillator circuitry in which body capacitance and resistance forms part of a variable impedance network whereby adjustment of the total impedance will diminish the A.C. voltage output when continuity exists to a level below that sufficient to actuate an alarm but when a break in continuity from the body to ground occurs will cause the A.C. output level to exceed the threshold necessary to trigger said alarm.
Other objects of this invention is to provide an improved device of the character described which is easily and economically produced, sturdy in construction and highly efficient and effective in operation.